Ink jet printers offer a mechanism for producing high print quality using inexpensive print materials. Typically, a print head includes a silicon substrate having hundreds of tiny jets per inch, each ejecting droplets of ink under the control of a microprocessor. This print head is usually mounted within a movable pen, which travels on a carriage directly over a paper conveyance path. In black-and-white printing, a single ink supply and print head is used, whereas two to four ink supplies and associated pens are normally used in color printing. Conventionally, in home printers, the ink supply is contained directly in each pen, and the pen usually must be completely replaced when the ink is gone. In larger ink jet printers used in some businesses, the ink supply is usually removed from the pen (so-called off-axis printing) due to the large ink supply required.
In both home and commercial applications, the cost of printing can be significantly affected by the need to occasionally replace the ink cartridge and its attached print head. This cost is somewhat lessened with off-axis printing since a relatively larger ink supply may be used (requiring less frequent ink replenishment) and may be more easily replaced using a removable, remote reservoir. Also, with the reservoir detached from the print head, the print head does not need to be replaced each time the ink reservoir is replaced.
One logistical problem in off-axis printing, however, is that it becomes more difficult to regulate the pressure of the ink supplied to the print head, sometimes called the pen "back pressure." Importantly, the ink near the print head is usually held slightly less than atmospheric pressure, to avoid any tendency of the ink to drool from ink jet spray nozzles. At the same time, a minimum ink pressure usually must be maintained in order to reliably print.
Air trapped inside the local ink compartment of a pen can present a significant problem in controlling back pressure. Air can become trapped due to a variety of causes: For example, air dissolved in ink can be reduced over time or through temperature changes; air can be introduced by shipping or priming procedures, or when an ink supply is replaced; air can enter the ink supply through the print head, or via diffusion through tubing or other pen components. Since air is much more compressible than ink and expands with temperature or altitude, a small change in the quantity of air present in an ink supply can dramatically affect print quality. Air bubbles can also potentially clog the tiny jets of a print head, thereby directly affecting print quality and print head life.
Some methods have been proposed for cleaning print heads or for purging air bubbles from print heads. These methods, while generally successful for their intended purposes, generally do not provide an effective mechanism for removing large quantities of air trapped inside an ink supply. Similarly, while some air could be deliberately used as a compliant element inside ink pens, the proposed methods of purging air, however, are also generally not sufficiently precise to control air quantities for this purpose.
A need exists for a system that can purge trapped air in a print mechanism. Further still, a need exists for a system which can purge air directly from a local ink reservoir, such that pen back pressure can be more precisely controlled. Ideally, such a system should permit precise control over air within a print mechanism, such that some air can be left in the print mechanism if desired for some pen designs. The present invention solves these needs and provides further, related advantages.